In general, a water treatment system using membranes is classified as an vacuum type system which soaks a membrane module in water to be treated and applies negative pressure to suction and filter the water or an external pressure type system which forcibly sends water to be treated to a casing filled with membranes.
The vacuum type system has advantages in that the system is simple, and treatment of a high-concentration, high-viscosity solution is easy, whereby pre-treatment processes, such as cohesion and precipitation, are unnecessary. Also, in a case in which the membranes are soaked so that the operation is performed as the vacuum type system, it is possible to completely remove particle materials from the finally treated water by the membranes and, in addition, to maintain the concentration of high-concentration microorganisms in a reaction vessel. Consequently, it is possible to obtain a higher contaminant decomposition effect through microorganisms than in an active sludge method.
However, the vacuum type system requires a reaction vessel to soak the membrane module. Also, the vacuum type system has a structurally limited space, and therefore, it is necessary to increase the size of the reaction vessel in increasing the size of the system due to enlargement of the system.
That is, in a real process to which the vacuum type membrane system is applied, the size of the initially designed reaction vessel is fixed. For this reason, when it is necessary to increase treatment capacity due to the increase of the amount of necessary treatment, construction of another reaction vessel is inevitable.
On the other hand, in the case of the external pressure type system, a membrane module is installed to filter water to be treated without a reaction vessel. This system has advantages in that, when a consumer needs greater treatment capacity, such a need is satisfied only by further installing a membrane module in an open space in addition to the reaction vessel. Consequently, it is possible to increase the size of the external pressure type system.
However, the external pressure type system has problems in that pre-treatment is necessary, and an amount of circulation necessary to maintain superficial flow velocity of the membranes is so large as to increase the size of a pump, whereby energy efficiency is lowered.